One of techniques for preserving foods, pharmaceutical products and the like is preservation utilizing oxygen absorbing agents (oxygen scavengers). Specifically, the preservation utilizing oxygen absorbing agents is a technique in which an oxygen scavenger capable of absorbing oxygen in atmosphere, together with an object, is placed within a hermetically sealable packaging body, and the inside of the hermetically sealable packaging body is brought to an oxygen-free state to prevent an oxidation-derived deterioration, a fungal deterioration, discoloration and the like of objects.
Oxygen scavengers formed of various inorganic materials and oxygen scavengers formed of various organic materials have hitherto been proposed for the removal of oxygen in atmosphere. Examples thereof include oxygen scavengers comprising inorganic main agents, for example, iron or other metal powders, sulfites, bisulfites, and dithionite, and oxygen scavengers comprising organic main agents, for example, L-ascorbic acid, erythorbic acid and salts thereof, saccharides such as glucose, and reducing polyhydric alcohols such as cathecol and pyrogallol.
These conventional oxygen scavengers, however, suffer from a problem that, in use, when a material that can supply water or moisture does not exist, an oxygen scavenging capability high enough to be used for practical use cannot be provided. Specifically, in conventional oxygen scavengers, an oxygen scavenging capability high enough to be used for practical use cannot be provided without mixing of the material with water or moisture retained thereon, for example, compounds containing water of crystallization, in use, or utilization of water vapor released from foods or the like to be preserved. Accordingly, difficulties have been encountered in applying conventional oxygen scavengers to pharmaceutical products or dried foods that should be used or preserved under drying conditions, or to storage of metal products that are weak against water or moisture without rusting.
Accordingly, oxygen absorbing agents that do not require moisture in oxygen absorption have been demanded in these applications. Oxygen absorbing agents reported as meeting this demand include, for example, oxygen scavengers comprising cerium oxide utilizing oxygen defects as a main agent (Japanese Patent Application Laid-Open No. 185653/2007), oxygen scavengers comprising titanium oxide containing oxygen defects as a main agent (Japanese Patent Application Laid-Open No. 104064/2005), oxygen scavengers comprising a metal subjected to hydrogen reduction as a main agent (Japanese Patent Application Laid-Open No. 277148/1987), and oxygen scavengers that utilize autoxidation of organic substances.
Among the above oxygen scavengers, oxygen scavengers disclosed in Japanese Patent Application Laid-Open No. 185653/2007 and Japanese Patent Application Laid-Open No. 104064/2005 utilize rare metals as starting metals that are rare and expensive. Further, the rare metals should be obtained through import from foreign countries, and, thus, depending upon further situation development, there is a possibility that the purchase of stock is varied, making it impossible to provide stable amount of production. Accordingly, these oxygen scavengers are not always satisfactory from the viewpoints of cost and stable supply of the rare metals. The oxygen scavenger disclosed in Japanese Patent Application Laid-Open No. 277148/1987 requires the provision of a large hydrogen reduction equipment in the production thereof and thus cannot be simply produced and, at the same time, cannot be said to have good handleability in the atmosphere. Furthermore, the oxygen scavenger utilizing autoxidization of the organic substance utilizes an oxidation reaction of the organic substance as the main agent, posing a problem of a by-product produced after oxygen absorption.
Accordingly, there is still a demand for an oxygen absorbing agent that can absorb oxygen in the atmosphere even in a moisture-free or substantially moisture-free atmosphere, is advantageous in stable availability of starting materials at low cost, is substantially free from the problem of the by-product, and is not necessary to provide a large apparatus for hydrogen reduction as an incidental equipment.